1. Field of the Invention
This invention relates to a bare electrode for welding of low temperature steel. More particularly, this invention relates to a bare welding electrode for use in a large current, gas shielded arc welding process in which steel for low temperature use, is welded with high efficiency. The electrode provides a joint having good properties, especially high notch toughness, and which can fully meet demands to improve the properties of welded joints. The need for improved welded joints is increasing with propagation of automation and high efficiency operation in welding processes.
2. Description of the Prior Art
In gas shielded arc welding, shielding gases are generally divided into three types, namely an inert shielding gas composed solely of an inert gas such as Ar, He or the like; an inert gas, in a broad sense, formed by adding an active gas such as O.sub.2, CO.sub.2 or the like to an inert gas, and a carbon dioxide shielding gas represented by CO.sub.2 or CO.sub.2 -O.sub.2. In gas shielded arc welding using a shielding gas, such as those mentioned above, when the inert gas-active gas ratio is changed in the shielding gas, as, the content of CO.sub.2 or O.sub.2 is increased, the oxygen content in the weld metal increases. The notch toughness of the weld metal is reduced as its oxygen content increases. More specifically, as is shown in FIG. 1, as the content of the active gas, such as CO.sub.2, O.sub.2 or the like increases in the inert shielding gas the oxygen content in the weld metal gradually increases. Further, as is shown in FIG. 2, the notch toughness of the weld metal is reduced as the content of the active gas such as CO.sub.2, O.sub.2 or the like in the shielding gas increases.
In the conventional gas shielded arc welding of steel, fine wires having a diameter of about 0.8 mm to 1.6 mm are generally used as the electrode and the maximum welding current applied is about 400 Amps. According to such conventional processes the notch toughness of the weld metal can be improved by reducing the amount of CO.sub.2, O.sub.2 or the like in the shielding gas. However, the conventional processes cannot meet the increasing demands to improve and enhance the operating efficiency and welding capacity. This is especially true in the case of steels for low temperature use, relatively thick plates are often used for satisfying low temperature demands which require products having excellent toughness at low temperatures.
As a means for meeting the foregoing demands, there can be mentioned the so called large current shielded arc welding process in which a large diameter wire (having a diameter of at least 3.0 mm) is used. The welding is performed by applying a large welding current so as to improve the efficiency in the gas shielded arc welding. In this welding process, however, the cooling rate of the weld metal is lower than in the gas shielded arc welding process using a small diameter wire, as a result the grain size of the weld metal is coarse. Also because of the difference of the arc phenomenon the quality of the weld metal cannot be improved merely by reducing the active gas content in the shielding gas, and the notch toughness of the weld cannot be improved at all.
One means for preventing the formation of coarse grain size in the weld metal, is by the known technique of adding Ti or Ti plus B to make the grain size finer and thereby improve the notch toughness (see, for example, Japanese Patent Publication No. 12258/68). However, in the large current inert gas shieled arc welding processes, the low temperature notch toughness is only slightly improved by the mere addition of such elements. Further, satisfactory results are not obtained when the above technique is applied to the large current inert gas shielded arc welding of steel for low temperature use. Accordingly, there exists the need for a wire suitable for use in large current gas shielded arc welding of low temperature steels.
The present inventors have conducted extensive research on the arc phenomenon and its metallurgical aspects in an attempt to overcome the defects of conventional large current inert gas welding processes for low temperature steels. As a result of this research it was found that in the large current, inert gas shielded arc welding processes using a large diameter wire, that the wire should contain Ti or Ti and B in suitable amounts together with suitable amounts of a deoxidizing element and at least one alloying element selected from Ni, Cr and Mo. Further, that when the strength of the weld metal is maintained at a level as low as possible in order to impart a sufficient plastic deformability under an impact load or tensile load to the weld metal, the welding can be performed at a high efficiency using a large current and the low temperature toughness is remarkably improved.